The invention relates generally to a method of and an apparatus for removing volatile and semi-volatile contaminants from a contaminated site in the earth and, more particularly, relates to a method of and an apparatus for removing volatile and semi-volatile contaminants from the earth by heating a portion of the contaminated earth by electromagnetic energy. The heated contaminated site is swept by air in a controlled manner to avoid interrupting the electromagnetic heating, to avoid fugitive emissions of contaminants to the atmosphere and to provide an air-contaminant vapor-water vapor mixture having a relatively high concentration of contaminant vapor easily adsorbed by a carbon bed or the like. An economic and efficient removal of the contaminants is achieved.
In recent years the public and the government have come to recognize that small amounts of various organic materials that have been spilled or improperly disposed of at various sites are environmental hazards and must be cleaned up. In the past, cleanup operations required that the entire site be excavated and that the soil and other site materials contaminated with unwanted or dangerous materials, such as hydrocarbons, halocarbons, and the like, had to have substantially all the contaminating material removed from them. In a typical prior art decontamination method, the entire site is excavated and all the excavated site materials are burned in a portable incinerator. Such a method is costly if the site is extensive, and may be impractical due to the large volume of contaminated soil if the contaminated site is deep.
Another method, as disclosed in U.S. Pat. No. 4,670,634 to Bridges, et al., treats a contaminated site by heating it with radio frequency energy. A plurality of fringing field electrodes is electrically excited by a radio frequency current to produce a bound, fringing, time-varying electric field that dielectrically heats portions of the contaminated site located below the electrodes. Volatile contaminants trapped in the site are volatilized or distilled and create sufficient autogenous pressure that they can be vented from the site surface into a chamber confined by a tent-like vapor barrier over the site. As may best be seen in FIGS. 1 and 3 of Bridges, et al., vapor exiting the ground is collected beneath the tent-like vapor barrier and carried by a vapor and gas collection line 34 to an external gas-liquid separator. In an alternative embodiment, as may best be seen in FIG. 7 of Bridges, et al., gravel is placed on the site surface in two layers. A plurality of gas and vapor collection ducts 68 is buried in the lower level of the gravel for carrying evolved contaminant vapors away from the site to treatment apparatus. The lower gravel layer, with the collection ducts positioned therein, is covered by an impermeable vapor barrier and the impermeable vapor barrier is overlaid by the upper gravel layer. The fringing field electrodes are positioned above the upper gravel layer substantially free from contact with it.
A similar system is disclosed in H. Der, et al., "In Situ Radio Frequency Heating Process for Decontamination of Soil," Solving Hazardous Waste Problems, presented at the 191st Meeting of the American Chemical Society, Apr. 18, 1986. The Dev, et al. system includes fringing field electrodes covered by the tent-like vapor barrier. Der, et al. also disclose that a site may be heated by radio frequency energy supplied by tubular vertical electrodes in boreholes, or by horizontal electrodes positioned above the surface of the soil to be heated. The electrodes are energized by a source of electrical energy producing an electric current having a frequency in the range of 6 MHz to 13 MHz. Transport of vaporized volatile contaminant to the collection region from the site is effected solely by the vapor pressure of the heated volatile contaminant and evolved water vapor. Dev, et al. disclose experimentation with small batches of sandy soil to determine the feasibility of removing chlorinated hydrocarbons, in particular tetrachloroethylene, from them. Dev et al. also discuss vacuum extraction technologies as alternatives for the Dev et al. system. The vacuum extraction technologies are directed to removing volatile contaminants from soil by drawing a vacuum inside or adjacent to the contaminated region of the site so that the contaminant is drawn out of the site.
In the methods described by Bridges, et al. and Dev, et al., it is necessary to heat the soil to a temperature sufficient to increase the vapor pressure of the contaminants to cause their volatilization and to overcome the pressure drop needed for movement of the volatilized contaminants through the contaminated earth to the collection region. Because the collection region is near the surface, it is not practical to draw a vacuum at the collection region greater than one inch on water gauge. As a result, pressures higher than atmospheric are generated in portions of the heated soil to overcome the pressure drop for movement of the volatilized contaminants from the region of volatilization to the collection region. Generation of such superatmospheric pressures may result in fugitive emissions of the volatilized contaminants from the earth's surface in regions not covered by the vapor-barrier, thereby contributing to air pollution.
As discussed above, vacuum or reduced pressure is used for in situ remediation of soils contaminated with hydrocarbons, such as solvents or fuels and is generally referred to as vacuum extraction technology, sometimes abbreviated as VET. A number of workers in the art are offering commercial remediation services based on this technology. The commercial methods and apparatus generally involve drilling a well into the vadose zone of the earth followed by the application of vacuum to volatilize and collect the contaminants. Multiple wells are sometimes used for large contaminated sites. Injection wells are used in combination with recovery wells in alternative methods. A common drawback is the inability to treat economically sites containing relatively less volatile materials, such as jet fuels.
U.S. Pat. No. 4,183,407 to Knopik discloses an underground exhaust system for removing vapors. The system employs a number of underground conduits inserted through an excavated shaft. A plurality of elongated and perforated collection elements connected to the conduits are buried in the contaminated site. An exhaust system for drawing gasoline vapors from the contaminated site is connected to the other end of the conduit. Such a system involves expensive shaft preparation, drilling of radial holes for conduits, and disposal of the excavated soil.
Visser and Malot U.S. Pat. Nos. 4,593,760 and 4,660,639 disclose vacuum extraction technology and well completion systems for decontamination of the vadose zone and for recovery of liquids trickling through the vadose zone. Their system effects vaporization, at ambient temperature, of the contaminants present in the vadose zone by applying sufficient vacuum. It is feasible to draw a vacuum sufficient to evaporate light solvents or volatiles, such as carbon tetrachloride or benzene, at ambient temperature. However, for most decontamination applications involving soils containing semi-volatiles or high boiling materials, such as jet fuels, the amount of vacuum needed for significant evaporation is relatively high. Visser and Malot depend upon putting a conduit in a larger borehole and completing it in such a way that the lower portion is filled with an permeable medium and the upper portion is filled with an impermeable medium. From practical considerations, the borehole cannot be larger than the conduit. This produces an annulus of limited dimensions.
Agrelot, Malot and Visser, Vacuum: Defense System For Ground Water VOC Contamination disclose use of a nearly complete vacuum, 29.9 inches of mercury, for sites contaminated with a light solvent such as carbon tetrachloride. Use of such a high vacuum, however, results in intrusion of large quantities of air through the soil and significant dilution of the contaminant vapor with air. The concentration of carbon tetrachloride in the air-carbon tetrachloride effluent is only about 2.7 per cent based upon the data of Agrelot, et al. Since carbon tetrachloride has a vapor pressure of over 100 millimeters of mercury at 25.degree. C. if no air bypass has occurred, the concentration of carbon tetrachloride in the effluent is expected to be about 13 per cent based upon thermodynamic considerations. The Agrelot, et al. data show that almost 80 per cent of the total volume of air is bypassing, it is not directly participating in vaporization of the contaminants. Use of such a system for soils containing semi-volatile materials, such as jet fuel, diesel fuel, chlorinated phenols and biphenyls, polynuclear aromatics, and creosote with vapor pressures of one millimeter of mercury or less at room temperature will produce effluents containing only one to one hundred parts per million of contaminants. Treatment of the effluents containing such low concentrations of contaminants, including steps of extracting the low concentration contaminant vapors from the air and contaminant vapor mixture, is economically prohibitive. The quantity of air to be moved through the vacuum system to decontaminate a site also is very large, due to the large quantity of contaminant in a typical site and the low concentration of the contaminants in the effluent stream and bypassing. This makes such methods expensive for such applications.
U.S. Pat. No. 4,442,901 to Zison discloses an apparatus and method for recovering methane and other gases from a landfill by using shallow wells having a reduced pressure placed thereon. A gas barrier covers a portion of the site to prevent air from breaking through into the methane containing regions and induce flow of landfill gas into the collector from regions radially outward from the collector. The gas barrier may be a thin plastic sheet or a polymerized clay such as polymerized bentonite and may be coextensive with or extend radially outwardly beyond the collector.
U.S. Pat. No. 4,730,672 to Payne discloses a closed-loop system that treats contaminants by a combination of condensers, a carbon bed, and reinjection of the cleaned effluents back into the vadose zone under pressure. The method suffers from the same drawback as those discussed above because it produces dilute effluents and injects air under pressure into the vadose zone which may result in fugitive emissions.
Another major difficulty of some of the vacuum extraction systems of the foregoing systems, such as Zison, is that subsurface air flow patterns are inhibited or are subject to bypassing in the region of the collector. The bypassing subsurface air flow patterns have a quasi-cylindrical or spherical symmetry about vertical perforated collectors. Thus, at the more distant radial positions from the vertical collectors the air flow is low, whereas close to the perforated vertical collectors the air flow is large. This results in nonuniform decontamination, partly because air flow rate varies through the treatment region, but also because the absolute pressure varies greatly within the treatment region. Near the collector the pressure will be low enough to vaporize a wide spectrum of contaminants, whereas at the more distant points, the absolute pressure will be nearly atmospheric, resulting in incomplete decontamination at the volumes more distant from the collector.
Similarly to Visser et al., U.S. Pat. No. 4,957,393 to Buelt, et al. discloses a system that also is relatively inefficient. In one version Buelt, et al. require the air to be removed by vacuum at an opening at the bottom of each electrode. Such an arrangement causes air flow to bypass the main portion of the site being heated. Air flows directly from outside the processing area into the bottom parts of the electrodes which are located around the periphery of the area being processed. At the same time, the vapor pressures of the materials desired to be removed are increased to a point where they may escape into the atmosphere. In another version, Buelt, et al. suggest the use of a hood under a vacuum which is placed over the processing area. Again the system is such that air can bypass the contaminated region by flowing around the edges of the hood and not being drawn through the deposit being heated. In fact, the system proposed by Buelt, et al. does not rely on the use of an air sweep to dry out the deposit, but rather it depends on raising the temperature of the deposit to a point where the vapor pressures of the contaminants drive them out of the soil.
Buelt, et al. also heat the waste site to temperatures well above the boiling point of water, but less than the melting point of the soil constituents, for an extended period of time in order to volatilize contaminant material. Thus, Buelt, et al. rely upon heating the soil well above the boiling point of water to generate sufficient vapor pressure such that the contaminants are easily collected.
Such high temperatures have the advantage of being able to remove not only the semi-volatiles but also a substantial fraction of refractory compounds. However, the large amount of energy needed not only to boil the water off, but also to heat the deposit to a suitable temperature whereat the contaminants themselves are distilled, volatilized or pyrolyzed to generate sufficient pressure to cause their migration through the soil to the contaminant collection system is very expensive. The contaminants also may be forced from the contaminated region by steam drive. Steam distillation reduces the vapor pressure of the contaminant but at the expense of added equipment. However, such auxiliary procedures introduce further equipment complexity.
U.S. Pat. No. 4,973,811 to Bass discloses a decontamination system employing eddy current or induction heating of a contaminated site by an above ground RF transmission line 21 and connected electrodes 12 and 14 excited by a high RF current from a constant current RF source 20. A vapor barrier 24 confines the contaminant emissions from the site and is connected to ducts 22 to carry the contaminant vapor to a mobile treatment system 23. In order to prevent condensation of contaminant vapor above ground, sweep air is supplied over the site and a radiant surface 26 above the electrodes 12 and 14 may also be employed. When the site is dried the heating mode may be switched from induction heating to fringe field heating.
Bass requires a radio frequency generator for energization of the electrodes 12 and 14 in both the induction mode as water is being evaporated and the fringe field heating mode when the water has been driven off. Bass is effective only for treating relatively shallow contaminated regions due to the decrease in the eddy current density at deeper points of the site remote from the above electrodes 14 and 14. The Bass system also may release fugitive emissions from under the edges of the vapor barrier 24. Bass does not suggest the use of below ground vacuum or controlled air sweep to speed the vaporization of the contaminant.
U.S. Pat. No. 4,984,594 to Vinegar, et al. discloses an in situ method for removing contaminants from surface and near surface soil by imposing a vacuum on the soil and includes a surface heater energized from a source of low frequency electrical energy at a frequency of 60 Hz by means of a common bus line. A pumping manifold pipe is connected to a vacuum collection system 16 and is also coupled to a highly permeable mat 22 which serves as a conduit for flow beneath an impermeable sheet.
The Vinegar, et al. system suffers from the problem that vacuum extraction does not take place from deep in the earth, nor is there heating from in the earth. The system is only effective for removing materials from the very top of a contaminated soil surface.